Non-aqueous electrolyte secondary batteries can achieve high voltage and high energy density. For this reason, they have been vigorously investigated. The positive electrodes of non-aqueous electrolyte secondary batteries under investigation comprise a transition metal oxide or transition metal chalcogenide such as LiMn2O4, LiCoO2, LiNiO2, V2O5, Cr2O5, MnO2, TiS2 or MoS2. These oxides and compounds have a layered or tunnel-like crystal structure through which lithium ions can migrate freely. The negative electrodes, on the other hand, comprise a carbon material capable of reversibly absorbing and desorbing lithium that is excellent in terms of cycle life and safety. Currently, lithium ion batteries whose negative electrode comprises a graphite-based carbon material are commercially available.
Graphite materials, however, have a relatively low theoretical capacity of 372 mAh/g and a relatively low theoretical density of 2.2 g/cm3. Accordingly, what is desired is a metal material for use in negative electrodes capable of achieving higher capacity than graphite materials. Among the candidate materials therefor, particularly, silicon (Si), which has a high capacity of 4199 mAh/g (theoretical density: 2.33 g/cm3), has been under extensive research and development.
Although Si is considered the most promising negative electrode material for achieving high capacity, the use of Si in a battery impairs the charge/discharge cycle characteristics of the battery. More specifically, during charge and discharge reactions, Si repeatedly expands and contracts along with the absorption and desorption of lithium. This increases the contact resistance among particles inside the negative electrode and degrades the current collecting network. The degradation of current collecting network can be a major factor in shortening charge/discharge cycle life.
In order to solve the above problems, various proposals have been made. For example, U.S. Pat. No. 6,090,505 (Patent Document 1) and Japanese Laid-Open Patent Publication No. 2004-103340 (Patent Document 2) propose, as a negative electrode material, an alloy material capable of reversibly absorbing and desorbing lithium comprising a solid phase A and a solid phase B having different compositions. In this alloy material, at least part of the solid phase A is covered with the solid phase B. The solid phase A comprises silicon, tin, zinc, etc. The solid phase B comprises a Group 2A element, a transition element, a Group 2B element, a Group 3B element, a Group 4B element, etc. The solid phase A is preferably amorphous or low crystalline.
Japanese Laid-Open Patent Publication No. 2004-335272 (Patent Document 3) discloses that improved cycle life can be attained by using a negative electrode material comprising at least two different phases: a phase A composed mainly of Si; and a phase B comprising a silicide composed of Si and a transition metal. This publication proposes that at least one of the phase A and the phase B is amorphous or low crystalline.
Japanese Laid-Open Patent Publication No. 2000-173616 (Patent Document 4) discloses that further improved cycle life can be attained by incorporating iron in the alloy in an amount of 0.002 wt % or more relative to the total amount of the alloy.